As a fastening structure for a motorcycle engine, for example, there exists an engine fastening structure in which a crankcase side flange portion of a cylinder body is fastened to a crankcase with bolts and a cylinder head side flange portion of the cylinder body is fastened to a cylinder head with bolts.
With the conventional construction, however, in the case of a single cylinder and large displacement engine that is subjected to a large load due to combustion pressure, the large load eventually generates a large tensile stress at an axially intermediate portion of the cylinder body.
Then, conventionally, it is a generally accepted practice to secure a required durability by increasing the thickness of the axially intermediate portion of the cylinder body. However, increasing the thickness of the cylinder body like this constitutes a cause for an increase in the weight of the engine.
On the other hand, as a conventional engine fastening structure which can avoid the increase in the engine weight, there exists, for example, an engine fastening structure disclosed in JP-A-8-28210. In this engine fastening structure, a crankcase side flange portion of a cylinder body 2 is fastened and fixed to a crankcase 3, 4 with case bolts 11, and a cylinder head side flange portion of the cylinder body 2 is fastened and fixed to a cylinder head with bolts 15. Furthermore, the cylinder head 1 is fastened and fixed to the crankcase 3, 4 with bolts 17 which screw through the cylinder body 2.
In the case of the engine fastening structure disclosed in the above publication, since the cylinder head 1 is fastened and fixed to the crankcase 3, 4 with the bolts 17 which screw through the cylinder body 2, part of a combustion pressure applied to the cylinder body is borne by the bolts 17, and stress generated in the cylinder body can be reduced accordingly, thereby making it possible to improve the durability of the cylinder body.
With the engine fastening structure disclosed in the publication, however, while the head bolts are screwed into the crankcase at positions which align with fixing positions of the cylinder head, since there exist cooling water jackets in the cylinder head, the head bolts have to be disposed outwardly so as to avoid the cooling water jackets. Due to this, as seen from the top, the crankcase is fastened at positions which are apart from the axis of a cylinder, and hence the crankcase has to be enlarged accordingly, which would other wise be unnecessary. In addition, the construction is adopted in which the head bolts are screwed into the crankcase, since the head bolts have to be disposed at positions where they do not interfere with a web of a crankshaft and the fixing positions of the cylinder head and fixing positions of the crankcase have to be aligned with each other, the degree of freedom in design is reduced.
The present invention was made in view of the problems inherent in the conventional engine fastening structure, and an object of the invention is to provide an engine fastening structure which can secure the durability of an engine without needing to enlarge a crankcase unnecessarily and without needing to reduce the degree of freedom in arrangement of head bolts.